1. Field of the Invention
The present invention generally relates to an AC driven, capacitive flat matrix display panel, that is, a thin film EL device. More particularly, it relates to a drive circuit therefor.
2. Description of the Prior Art
By way of example, a double insulated (or triple-layered) thin film EL element is constructed in the following manner.
As shown in FIG. 6, the EL element shown therein comprises a glass substrate 101 having a group of strip-shaped transparent electrodes 102 made of In.sub.2 O.sub.3 and deposited on one surface thereof in parallel relationship with each other. It includes a three-layered structure including a layer 103 of dielectric material such as, for example, Y.sub.2 O.sub.3, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3, an EL layer 104 made of ZnS doped with an activator such as, for example, Mn and a layer 103' of dielectric material such as, for example, Y.sub.2 O.sub.3, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3. This layer is sequentially formed over the group of the strip-shaped transparent electrodes 102 to a film thickness of 500 to 10,000 .ANG. by the use of a thin film technology such as, for example, a vapor-deposition technique or a sputtering technique. Further, a group of strip-shaped counter-electrodes 105 made of aluminum are deposited over the three-layered structure in parallel relationship with each other so as to extend in a direction perpendicular to the group of the strip-shaped transparent electrodes 102.
The thin film EL element of the above described construction comprises a sandwich structure. The EL layer 104 is sandwiched between the dielectric layers 103 and 103' and is disposed between the groups of the electrodes 102 and 105. Thus, the thin film EL element in question can be considered an equivalent of a capacitance element. Also, as can be readily understood from the graph of FIG. 7 illustrating the voltage versus luminance characteristic, this thin film EL element of the above described construction is adapted to be driven by the application of a relatively high voltage, for example, about 200V. A further feature featured is that it can be energized by an alternating current field to emit rays of light of high luminance. Thus, it has a long liftime.
Hitherto, various drive circuits for the thin film EL element of the above described type have been proposed for the purpose of minimizing the consumption of an electric modulating power, the manufacturing cost and the thickness and/or size thereof.
In any one of the proposed drive circuits, it is a general practice to connect scanning electrodes and data electrodes with driver IC circuit having only a push-pull function or a pull-up, pull-down function. These switching elements are employed in the form of Nch MOS field-effect transistors, Nch transistors, Pch MOS field-effect transistors or Pch transistors. This is because they can contribute to the minimization of the manufacturing cost and the bulkiness and also to the accomplishment of a highly integrated feature.
It has, however, been found that, since these switching elements provide an output of low current and a high ON resistance, a phenomenon tends to occur wherein, in the EL display device, the lighting luminance tends to vary depending on the number of picture elements energized to light during the drive of one scanning line. This phenomenon is illustrated in FIG. 4 which illustrates a display screen of the EL display device. In FIG. 4, hatched bars 11 represents non-lighting portions and reference numeral 12 represents a lighting portion. Assuming that the lighting luminances at points (1), (2), (3) and (4) are expressed by B1, B2, B3 and B4, respectively, the luminances at these points have the following relationship. EQU B1&gt;B2&gt;B3&gt;B4
In other words, the increase of the number of picture elements which are energized to light results in reduction of the luminance. Therefore, the display quality tends to be lowered.
Where a gradation is desired to be displayed, this phenomenon cannot be neglected because it brings about a considerable adverse effect on the gradation display. By way of example, if the luminance at the point (4) is nine tenths (9/10) of the luminance at the point (1), the level of gradation at the point (1) which is 90% and the level of gradation at the point (4) which is 100%,are generally equal to each other. Therefore, a normal gradation display is impossible.
Also, when the display device is desired to be able to display a relatively large amount of information, that is, when the load capacitance of one scan line is desired to be increased as a result of the increased number of the data electrodes and the length of time required to accomplish one scan drive is desired to be reduced as a result of the increased number of the scanning electrodes, sufficient writing pulses cannot be applied because of the limitation imposed by the output current capacity and the ON resistance. Accordingly, no sufficient lighting luminance can be obtained.
As hereinabove discussed, there has been a problem associated with the improvement in display quality exhibited by the thin film EL display device.